The present invention relates to computed tomography (CT) imaging apparatus; and more particularly, to the gating of CT systems during cardiac scans.
In a current computed tomography system, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, termed the “imaging plane.” The x-ray beam passes through the object being imaged, such as a medical patient, and impinges upon an array of radiation detectors. The intensity of the transmitted radiation is dependent upon the attenuation of the x-ray beam by the object and each detector produces a separate electrical signal that is a measurement of the beam attenuation. The attenuation measurements from all the detectors are acquired separately to produce the transmission profile.
The source and detector array in a conventional CT system are rotated on a gantry within the imaging plane and around the object so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements from the detector array at a given angle is referred to as a “view” and a “scan” of the object comprises a set of views made at different angular orientations during one revolution of the x-ray source and detector. In a 2D scan, data is processed to construct an image that corresponds to a two dimensional slice taken through the object. The prevailing method for reconstructing an image from 2D data is referred to in the art as the filtered backprojection technique. This process converts the attenuation measurements from a scan into integers called “CT numbers” or “Hounsfield units”, which are used to control the brightness of a corresponding pixel on a cathode ray tube display.
Cardiac CT scanning is on the verge of revolutionizing medical diagnosis. Using multi-ring detectors, CT scans produce phenomenal images of the human heart, and using contrast agent injection, it is possible that coronary artery angiograms, the gold standard for coronary artery disease diagnosis, can be generated from coronary arterial CT scans with intravenous contrast agent injections. This, in essence, could replace catheter-based coronary angiograms, a highly invasive angiographic procedure, with a relatively benign, non-invasive diagnostic study.
The implications of this are huge. However, the CT technique is presently limited. The main problem is that reconstruction of 3D CT coronary angiograms are not good enough or reproducible enough to meet the imaging standards necessary to replace standard coronary angiography. For instance if one scans an ex-vivo, non-beating heart, it is possible to generate exquisite images of the coronary arteries with resolutions of tens of microns. However, using standard ECG gating in the beating heart, even with intravenous contrast injections, such resolutions cannot be approached. Yet, this is the type of resolution required in order to replace coronary angiography, which can have resolutions on the order of 50-1200 microns.
Multiple studies have documented the difficulties inherent in standard ECG gating. Many of these studies were performed to quantitate coronary arterial calcifications, and there seems to be a consensus that the major contributor to the variability in coronary artery calcification quantification, a measure of CT's inconsistency, is motion artifact. The present solution is ECG gating, yet the variables that affect gated reconstructions are many and include heart rate, scanning rate, and the fraction of the ECG over which gating is performed. The optimal time for gating seems to depend on the part of the heart/coronary tree being imaged. This is not surprising, since there is a known muscular contraction wave through the heart. Very slow heart rates (<50 beats/min) generally have the fewest artifacts but even then, if one picks the wrong portion of the R-R interval (90%), 75% of scans will still have artifacts. Higher, more typical heart rates, are more problematic with scans produced from hearts beating at such standard rates of 71-80 beats/min have about 35% of images on average degraded by artifacts. In general, it appears that early trigger times, 40% of the RR interval, have fewer artifacts than later times, 80% of the RR interval. Other studies have shown that the best image quality required heart rates less than 74.5 beats/min with the optimal fractional delays in the RR interval varying from 50% for the right coronary artery and 60% for the left. If the heart rate is too rapid, beta blockers must be used to slow the heart rate down.
In order to avoid gating, some authors have suggested that acquisition rates of less than 50 msec per slice are necessary to generate artifact free images. Although the fastest present acquisition rates by electron beam CT scanners can approach this limit, 100 msec per slice, the image quality of electron beam scanners does not match that of the standard mechanical scanners, making their faster speeds of limited advantage. The present multidetector array helical CT scanners typically operate at about 150-250 msec per slice. Ultimately, this all again means that gating is still a requirement.
Unfortunately, ECG gating has an inherent, intrinsic problem, which is that ECG gating is based on the heart's changing electrical potential, whereas the activity being gated is the heart's physical motion. Clearly, the electrical gating must be correct on average, because the heart's electric signal induces the motion, and we know that there is no net motion of the heart because, frankly, the heart remains in the chest. However, at any given time, the correspondence from beat to beat must undoubtedly fluctuate producing variations around this mean position. When grossly imaging the heart, these slight variations are probably OK. However, when near perfect precision is required in the reconstruction, even minor fluctuations will cause unacceptable degradations.
In order to overcome the limitations of ECG gating, other gating methods have been attempted. We previously performed a study by using an ultrasound range-gated Doppler device where we cardiac gated MRI scans directly from the motion of a volunteer's left ventricular wall or from the blood flow in a volunteer's right innominate artery using a CW Doppler. We were able to cardiac gate by feeding the audio-output of the Doppler device into a function generator that produced a 5V transistor-transistor logic (TTL) pulse that was fed into a ECG simulation circuit that was then fed into the gating circuitry of the MR imager.